Method of installing stroke sensor

ABSTRACT

A method of installing a stroke sensor that enables the stroke sensor to be adjusted in a simple process is provided. The method has the steps of: arranging a second magnet, relative to the magnetic field detecting element, at a physically determinable first reference position and obtaining an indicator value S1; attaching the first magnet and the magnetic field detecting element to structures different from each other, respectively, and positioning the first magnet, relative to the magnetic field detecting element, at a physically determinable second reference position, and obtaining an indicator value S2, wherein the second reference position corresponds to the first reference position; calculating ΔS=S1−S2, wherein ΔS is a difference between the indicator value S1 and the indicator value S2; and modifying a process in the processor such that a sum of the indicator value S and ΔS is outputted.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present application is based on, and claims priority from,JP2020-18512, filed on Feb. 6, 2020, the disclosure of which is herebyincorporated by reference herein in its entirety.

The present invention relates to a method of installing a stroke sensor.

2. Description of the Related Art

A stroke sensor is used in various kinds of fields, such as atransmission and a brake of a vehicle. JP2009-44888 discloses anactuator having a sensor for detecting a rotational angle. The sensor ispre-installed in the actuator. In order to compensate the error ininstalling the actuator, the moving distance of the drive shaft of theactuator and the sensor output are measured by operating the actuator,and the sensor characteristics that are stored in the sensor arerewritten accordingly.

SUMMARY OF THE INVENTION

In the method of rewriting the sensor characteristics disclosed inJP2009-44888, it is necessary to move the actuator after the sensor isinstalled in the actuator. As a result, the processes are complicatedafter the sensor is installed in the actuator.

The present invention aims at providing a method of installing a strokesensor that enables the stroke sensor to be adjusted in a simpleprocess.

According to the present invention, a method of installing a strokesensor is provided, wherein the stroke sensor includes a magnetic fielddetecting element that detects a magnetic field, a first magnet thatgenerates the magnetic field and that is movable in a first directionrelative to the magnetic field detecting element, and a processor thatcalculates an indicator value S based on the magnetic field that isdetected by the magnetic field detecting element, wherein the indicatorvalue S indicates a relative position of the first magnet relative tothe magnetic field detecting element. The method comprises the steps of:arranging a second magnet, relative to the magnetic field detectingelement, at a physically determinable first reference position andobtaining an indicator value S1; attaching the first magnet and themagnetic field detecting element to structures different from eachother, respectively, and positioning the first magnet, relative to themagnetic field detecting element, at a physically determinable secondreference position, and obtaining an indicator value S2, wherein thesecond reference position corresponds to the first reference position;calculating ΔS=S1−S2, wherein ΔS is a difference between the indicatorvalue S1 and the indicator value S2; and modifying a process in theprocessor such that a sum of the indicator value S and ΔS is outputted.

According to the present invention, it is possible to provide a methodof installing a stroke sensor that enables the stroke sensor to beadjusted in a simple process.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description withreference to the accompanying drawings which illustrate examples of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the arrangement of a stroke sensoraccording to an embodiment of the present invention;

FIGS. 2A to 2C are conceptual views showing the relationship between thedisplacement of the stroke sensor and the output voltage;

FIG. 3 is an example of a conversion formula used for the calibration ofa stroke sensor;

FIG. 4 is a view illustrating the influence of the positional deviationon the measurement accuracy;

FIGS. 5A to 5B are views showing a method of adjusting a stroke sensor;

FIG. 6 is a schematic view showing the arrangement of a stroke sensoraccording to a modification; and

FIG. 7 is a schematic view showing the arrangement of a stroke sensoraccording to another modification.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, a method of installing stroke sensor 1according to an embodiment of the present invention will be described.In the following descriptions, the direction in which the first magnetis moved is referred to as a first direction or the X direction. Thedirection that is perpendicular to the X direction and that isperpendicular to the surface of first magnet 3 that faces magnetic fielddetecting element 21 is referred to as the Y direction. The term“relative position” or “relative position of first magnet 3” refers tothe position of first magnet 3 in the X direction relative to magneticfield detecting element 21 unless otherwise defined.

FIG. 1 shows a schematic view of stroke sensor 1 that is installed on avehicle or the like. Stroke sensor 1 has sensor assembly 2 that includesmagnetic field detecting element 21 that detects a magnetic field andprocessor 22, as well as first magnet 3 that generates a magnetic fieldthat is detected by magnetic field detecting element 21. First magnet 3includes three sub-magnets 31 that are aligned in the X direction at thesame interval and yoke 32 that connects sub-magnets 31 to each other.The N pole and the S pole are alternately arranged on the surfaces ofthree sub-magnets 31 that face magnetic field detecting element 21.First magnet 3 is supported by first structure 33, such as a supportplate, and first structure 33 is connected to a movable element, such asa brake pedal (not illustrated). First magnet 3 is movable in the Xdirection relative to sensor assembly 2, that is, magnetic fielddetecting element 21. By detecting the position of first magnet 3relative to magnetic field detecting element 21, it is possible todetect, for example, the amount of depression of a brake pedal. A pairof stoppers 4 is provided on both sides of first structure 33, and firststructure 33 is movable between both stoppers 4. In other words, themovable range of first structure 33 is the range between both stoppers4, and the movable range of first magnet 3 is determined accordingly.

Sensor assembly 2 is attached to second structure 24 that is differentfrom first structure 33. Second structure 24 is, for example, a coverelement that covers first structure 33 and, second structure 24 ismovable in the X direction relative to first structure 33. In order toarrange magnetic field detecting element 21 near first magnet 3,protrusion 23, that has magnetic field detecting element 21 formed atthe tip end thereof, is inserted into hole 25 that is provided in secondstructure 24. Sensor assembly 2 has housing 26 that covers magneticfield detecting element 21 and processor 2. If second structure 24 has aspace for magnetic field detecting element 21 and processor 22, thenhousing 26 may be omitted. Magnetic field detecting element 21 includesan element that detects a magnetic field in the X direction and anelement that detects a magnetic field in the Y direction (both notillustrated). The type of these elements is not limited, and a Hallelement, a TMR element, a GMR element and so on may be used. Sensorassembly 2 is preferably immovable because a power cable, an outputcable and the like are connected to sensor assembly 2. Accordingly,sensor assembly 2 is immovable, and first magnet 3 is movable in thepresent embodiment. However, sensor assembly 2 may be movable, and firstmagnet 3 may be immovable.

Processor 22 calculates indicator value S that indicates the relativeposition of first magnet 3 based on the magnetic field that is detectedby magnetic field detecting element 21. Three sub-magnets 31 generate amagnetic flux having a substantially sinusoidal shape across them.Assume that the X component and the Y component of a magnetic flux at acertain position is Bx and By, respectively. Then, angle θ of themagnetic flux relative to the X direction at the position (hereinafter,referred to as angle θ of the magnetic field) can be expressed by arctan(By/Bx). The magnetic field around first magnet 3 can be obtained byanalysis etc. in advance. Since the gap between magnetic field detectingelement 21 and first magnet 3 in the Y direction is known, if angle θ ofthe magnetic field is obtained, then the relative position of firstmagnet 3 can be calculated. Processor 22 calculates angle θ of themagnetic field=arctan (By/Bx) from Bx and By that are detected bymagnetic field detecting element 21, converts angle θ of the magneticfield to output voltage V that corresponds to the relative position, andoutputs output voltage V. Angle θ of the magnetic field can be detectedover the angle range of 0 to 360°. Therefore, indicator value S isoutput voltage V that is determined based on angle θ of the magneticfield.

The relationship between the relative position and output voltage V istypically curvilinear and can be expressed by a curved line thatresembles a cubic function, as shown by the solid line in FIG. 2A.Accordingly, before stroke sensor 1 is installed in an actual apparatus,calibration to convert the relationship between the relative positionand output voltage V (the initial indicator value) to a linearrelationship is performed for each stroke sensor 1. The method of thecalibration will be now described.

First, sensor assembly 2 is attached to calibration apparatus 6 (seeFIGS. 4, 5A, 5B). Master magnet 5 (a second magnet) that is differentfrom first magnet 3 is attached to calibration apparatus 6. Calibrationapparatus 6 simulates first structure 33 to which first magnet 3 isattached and second structure 24 to which sensor assembly 2 is attached.Master magnet 5 has the same magnetic properties and shape as firstmagnet 3. Therefore, calibration apparatus 6 has substantially the sameconfiguration as the actual apparatus that is shown in FIG. 1 except forerror factors, such as manufacturing errors. The movable range of mastermagnet 5 of calibration apparatus 6 is set to be wider than the movablerange of first magnet 3 of an actual apparatus. Use of such calibrationapparatus 6 enables many sensor assemblies 2 to be efficientlycalibrated. Further, since sensor assembly 2 can be combined with anyfirst magnet 3 that is substantially the same as master magnet 5, themanagement of first magnet 3 can be easily performed. Calibrationapparatus 6 calculates angle θ of the magnetic field=arctan (By/Bx)while moving master magnet 5 relative to sensor assembly 2 over theentire movable range in the X direction, converts angle θ of themagnetic field to output voltage V that corresponds to the relativeposition, and outputs output voltage V. Since master magnet 5 and firstmagnet 3 are substantially the same, the relationship between therelative position and output voltage V is expressed by a curved linethat is substantially the same as the solid line in FIG. 2A.

Next, a line that connects output voltages V1 and V2 at both ends of themovable range is calculated. The line is regarded as the relationshipformula between the relative position and output voltage V that has beencalibrated (hereinafter, referred to as output voltage W). Therelationship formula between the relative position and output voltage Wis a linear function. In order to obtain the relationship formula,processor 22 has converting means that converts angle θ of the magneticfield to output voltage W. The converting means are generated as aconversion map shown in FIG. 3. For convenience, the converting meansare shown by a graph in FIG. 3, but the converting means are actuallystored as a table in the memory of processor 22. Conversion map isgenerated as a set of θ₀, θ₁, θ₂, . . . , θ_(N-2), θ_(N-1), θ_(N) versusoutput voltage W₀, W₁, W₂, . . . , W_(N-2), W_(N-1), W_(N) (whereΔθ=θ_(i)−θ_(i-1)=360°/N) (N is a natural number). N is selected, forexample, from the range between 30 and 40. In the present embodiment,the converting means convert angle θ of the magnetic field to outputvoltage W, but output voltage V may be converted to output voltage W.

Stroke sensor 1 works in the following manner. Magnetic field detectingelement 21 of sensor assembly 2 detects Bx and By. Processor 22calculates angle θ of the magnetic field=arctan (By/Bx). Processor 22converts angle θ of the magnetic field to output voltage W, thatcorresponds to the relative position, by means of the converting means,and outputs output voltage W. Angle θ of the magnetic field iscalculated from the conversion map by interpolation, as needed. In thismanner, stroke sensor 1 outputs output voltage W that is in a linearrelationship with the relative position.

However, there is a possibility that when first magnet 3 is attached tofirst structure 33 and sensor assembly 2 that has been calibrated isattached to second structure 24, the relationship between the relativeposition and output voltage W, that is, the relationship shown by thedashed line in FIG. 2A is not maintained. For example, due to themanufacturing tolerance that both first structure 33 and secondstructure 24 have, the positions of first magnet 3 and sensor assembly 2may be relatively shifted in the X direction. The position of hole 25 ofsecond structure 24 may also be shifted in the X direction. Thepositional shift may also occur when first magnet 3 is attached to firststructure 33 or when sensor assembly 2 is attached to second structure24. For example, due to a gap that is present between protrusion 23 ofsensor assembly 2 and hole 25 of second structure 24, it is difficult toprecisely control the position where sensor assembly 2 is attached tosecond structure 24. The positions of sensor assembly 2 and first magnet3 may also be shifted when these are attached by an adhesive and thelike.

FIG. 4 conceptually illustrates the influence of the positional shift onthe measurement accuracy. As shown in the upper part in FIG. 4, assumethat the calibration has been performed when protrusion 23 of sensorassembly 2 was attached, with protrusion 23 being concentric with hole25. On the other hand, in an actual apparatus shown in the middle partin FIG. 4, protrusion 23 of sensor assembly 2 was attached, withprotrusion 23 being in contact with the right edge of hole 25. Firstmagnet 3 is attached at the same position as master magnet 5. In otherwords, the position of sensor assembly 2 relative to first magnet 3 andthe position of sensor assembly 2 relative to master magnet 5 areshifted by distance D. In this case, sensor assembly 2 that has beeninstalled in the actual apparatus recognizes that first magnet 3 at theposition in the middle part in FIG. 4 is positioned as shown by thedashed line in the bottom part in FIG. 4. That is, sensor assembly 2recognizes that first magnet 3 is shifted rightward from the actualposition by distance D. In other words, when the end of first magnet 3is at position X1 that is shifted rightward from position X0 by distanceD, sensor assembly 2 outputs output voltage W that is to be outputtedwhen the end of first magnet 3 is at position X0. As a result, theoutput line is translated by distance D, as shown in FIG. 2B, and themeasurement accuracy of stroke sensor 1 worsens. It should be noted thatthe position of sensor assembly 2 relative to first magnet 3 and theposition of sensor assembly 2 relative to master magnet 5 may be shiftedfrom each other in the Z direction (the direction perpendicular to boththe X direction and the Y direction in FIG. 1). However, the Z directionis not a direction in which stroke sensor 1 detect a position, and thedistribution of the magnetic field does not largely vary in the Zdirection. Therefore, the positional shift in the Z direction does notlargely affect the measurement accuracy. The measurement error in the Zdirection is smaller, on the level of one digit, than the measurementerror that is caused by the positional shift on the same level as in theX direction. The measurement error when the position of sensor assembly2 relative to first magnet 3 and the position of sensor assembly 2relative to master magnet 5 are shifted from each other in the Ydirection is about the same level as the measurement error that iscaused by the positional shift in the Z direction.

Accordingly, the process that is carried out in processor 22 of strokesensor 1 is modified in the present embodiment. First, as shown in FIG.5A, the left end of master magnet 5 is arranged at first referenceposition XR1 in calibration apparatus 6. XR1 is a physicallydeterminable relative position. “Physically determinable” means that theposition of master magnet 5 in the X direction relative to magneticfield detecting element 21 can be determined by any physical means, suchas mechanical means, electric means and magnetic means. In the presentembodiment, the movable range of first structure 33 is delimited bymotor 35 that drives first structure 33. Therefore, the left end of themovable range of first structure 33 that is determined by the control ofmotor 35 is a position that can be determined by electric means.Examples of a position that is determined by mechanical means include aposition of a stopper that is provided in calibration apparatus 6 andthat simulates stopper 4 of an actual apparatus, a position where aspring is compressed to its maximum and so on. In the embodiment, firstreference position XR1 is arranged in the movable range of firststructure 33 and is spaced distance A rightward from left end position Pthat is delimited by motor 35. Position P is the position of leftstopper 4 of an actual apparatus, and the position of left stopper 4 issimulated by motor 35. Since master magnet 5 is precisely attached tofirst structure 33 at a predetermined position thereof, distance A ishighly accurate. Next, processor 22 calculates indicator value S1 thatindicates first reference position XR1 and outputs output voltage W1that corresponds to indicator value S1. Indicator value S1 is outputtedto adjustment apparatus 7 or a to storage apparatus for the laterprocess. These operations are performed as a part of the calibrationmentioned above. Since indicator value S1 that indicates first referenceposition XR1 is obtained in the calibration, no additional operation isrequired to obtain indicator value S1.

Next, as shown in FIG. 5B, first magnet 3 is attached to first structure33, and sensor assembly 2 is attached to second structure 24. In FIG.5B, in the same manner as shown in FIG. 4, sensor assembly 2 is attachedto second structure 24, with protrusion 23 being in contact with theright edge of hole 25. However, it is not necessary that the position ofsensor assembly 2 relative to hole 25 is known. In the followingprocess, adjustment apparatus 7 is used to modify the process ofprocessor 22 of stroke sensor 1. First magnet 3 is arranged at secondreference position XR2, which is a relative position that corresponds tofirst reference position XR1 and that is physically determinable. Theterm “corresponds to first reference position XR1” means that, based onthe entire arrangement of first structure 3 and second structure 24,second reference position X2 in the actual apparatus corresponds tofirst reference position XR1 in calibration apparatus 6. In the presentembodiment, second reference position XR2 is one end of the movablerange of the first magnet 3, that is, the position of the left end offirst magnet 3 when first structure 33 abuts against stopper 4 on theleft side, and second reference position XR2 corresponds to firstreference position XR1. In other words, in the present embodiment, firstreference position XR1 is positioned at the end of the movable range ofmaster magnet 5 in calibration apparatus 6 (that simulates the movablerange of first magnet 3 in the actual apparatus), and second referenceposition XR2 is positioned at the corresponding end of the movable rangeof first magnet 3 in the actual apparatus.

Next, processor 22 calculates indicator value S2 that indicates secondreference position XR2, and adjustment apparatus 7 reads output voltageW2, that is indicator value S2. Unlike the calibration, it is notnecessary to move first magnet 3. The left end of first magnet 3 staysat second reference position XR2. Second reference position XR2 may beat any position as long as the relationship between first referenceposition XR1 and second reference position XR2 can be definitelydetermined, but preferably at the initial position where theinstallation is performed. By doing so, it is not necessary to furthermove first magnet 3 after first magnet 3 is attached.

Next, adjustment apparatus 7 calculates a difference between indicatorvalue S1 and indicator value S2, i.e., ΔS=S1−S2. Indicator value S1 hasbeen directly inputted to adjustment apparatus 7 or has been inputted inadvance to adjustment apparatus 7 via the above-mentioned storageapparatus. As mentioned above, since the position of sensor assembly 2in calibration apparatus 6 is shifted from that in the actual apparatus,indicator value S2 is different from indicator value S1. Accordingly,adjustment apparatus 7 modifies the flow of processor 22 such that a sumof indicator value S and ΔS is outputted. Specifically, adjustmentapparatus 7 updates the program that is stored in processor 22 so as toadd ΔW=W1−W2 to output voltage W that is calculated by theabove-mentioned method, and to output the sum. For example, when outputvoltage W1 is 4.0V and output voltage W2 is 3.9V, stroke sensor 1 mustoutput 4.0V as output voltage W2 at second reference position XR2. Thisis because stroke sensor 1 is adjusted so as to output 4.0V as voltageW1 at first reference position XR1 that corresponds to second referenceposition XR2. If output voltage W1 of 4.0V is not outputted at secondreference position XR2, then the position of first magnet 3 cannot beprecisely detected. Accordingly, adjustment apparatus 7 modifies theprocess in processor 22 so as to add ΔW=4.0−3.9=0.1V to output voltageW2 of 3.9V and to output 4.0V as output voltage W2. Alternatively, theconversion map may be directly updated by a writing apparatus such thatoutput voltage W is increased by ΔW.

In this manner, as shown in FIG. 2C, the positional shift that occurswhen the sensor is installed in an actual apparatus is cancelled, andthe relationship between the relative position and output W that isobtained in the calibration is restored. This is because the positionalshift is generally so small that, by applying ΔS that is obtained at areference position to the other positions, the positional shift at theother positions can also be cancelled at about the same level. In otherwords, the measured value and the ideal value in FIG. 2B aresubstantially in the relationship of a translation. Thus, by determiningthe amount of the positional shift at one point and by translating thegraph of the measured values by the amount, the whole measured valuescan be modified with the same level of accuracy.

In the present embodiment, it is not necessary to move first magnet 3when stroke sensor 1 is adjusted in an actual apparatus. All that isrequired after first magnet 3 and sensor assembly 2 are installed in anactual apparatus is to obtain indicator value S2 at the initial positionwhere first magnet 3 and sensor assembly 2 are installed, to calculateΔS, and to modify the process in processor 22 accordingly. In general,the calibration of stroke sensor 1 is performed by the manufacturer ofstroke sensor 1, but the adjustment of stroke sensor 1, after it isinstalled in an actual apparatus, is carried out by the manufacturer ofan assembly to which stroke sensor 1 is incorporated or by themanufacturer of the final product. Accordingly, in the presentembodiment, the process that is performed by the manufacturer of theassembly or by the manufacturer of the final product is simplified andthe added value of stroke sensor 1 is increased.

In the present embodiment, a highly precise calibration can be performedeven when first magnet 3 is movable to limited positions, that is, onlydiscretely movable. For example, first structure 33 may be connected toan element, such as a plunger, that is configured to stop only at bothends of the movable range. When calibration of stroke sensor 1 iscarried out after stroke sensor 1 is installed in an actual apparatus,the measurements that can be used for the calibration are limited to thevalues that are measured at the two points. In the present embodiment,since calibration using measurements at multiple points is possiblebefore stroke sensor 1 is installed in an actual apparatus, the accuracyof the calibration can be improved.

An embodiment of the present invention has been described, but thepresent invention is not limited to the embodiment. For example, firstmagnet 3 may be used as second magnet 5 for the calibration. Firstmagnet 3 has substantially the same magnetic properties as master magnet5, but there is a possibility that the magnetic properties of the twomagnets are not completely the same. Since first magnet 3 that is to beinstalled in an actual apparatus is used for the calibration instead ofmaster magnet 5, measurement error that is caused by the difference inthe magnetic properties between first magnet 3 and master magnet 5 doesnot occur.

As shown in FIG. 6, first magnet 3 may be a single magnet. The intensityof magnetic flux By in the Y direction that is detected by magneticfield detecting element 21 changes depending on the relative position offirst magnet 3. Accordingly, the intensity of magnetic flux By can beused as an indicator value of the relative position of first magnet 3.In other words, in this modification, indicator value S is an outputvoltage that is determined based on the intensity of a magnetic field.

As shown in FIG. 7, if first magnet 3 can be stopped at intermediatepositions of the movable range by latch mechanism 34 or the like, thenthe intermediate positions where first magnet 3 can be stopped may beused as first reference position XR1 and second reference position XR.In other words, physically determinable first reference position XR1 andsecond reference position XR2 are not limited to both ends of themovable range.

In addition, the calibration may be omitted. As shown in FIG. 2A, it ispreferable for the output characteristics of stroke sensor 1 tolinearize the relationship between the relative position and the outputvoltage by the calibration, but linearization is not essential. Omittingthe calibration leads to simplification of the production process.

Although a certain preferred embodiment of the present invention hasbeen shown and described in detail, it should be understood that variouschanges and modifications may be made without departing from the spiritor scope of the appended claims.

What is claimed is:
 1. A method of installing a stroke sensor, whereinthe stroke sensor includes a magnetic field detecting element thatdetects a magnetic field, a first magnet that generates the magneticfield and that is movable in a first direction relative to the magneticfield detecting element, and a processor that calculates an indicatorvalue S based on the magnetic field that is detected by the magneticfield detecting element, wherein the indicator value S indicates arelative position of the first magnet relative to the magnetic fielddetecting element, the method comprising the steps of: arranging asecond magnet, relative to the magnetic field detecting element, at aphysically determinable first reference position and obtaining anindicator value S1; attaching the first magnet and the magnetic fielddetecting element to structures different from each other, respectively,and positioning the first magnet, relative to the magnetic fielddetecting element, at a physically determinable second referenceposition, and obtaining an indicator value S2, wherein the secondreference position corresponds to the first reference position;calculating ΔS=S1−S2, wherein ΔS is a difference between the indicatorvalue S1 and the indicator value S2; and modifying a process in theprocessor such that a sum of the indicator value S and ΔS is outputted.2. The method according to claim 1, wherein the second referenceposition is at an end of a movable range of the first magnet, and thefirst reference position is at a corresponding end of a movable rangethat simulates the movable range.
 3. The method according to claim 1,wherein a relationship between the relative position and the indicatorvalue S is expressed by a linear function.
 4. The method according toclaim 3, further comprising the steps of: obtaining a relationshipbetween relative positions of the second magnet and initial indicatorvalues while moving the second magnet in the first direction relative tothe magnetic field detecting element, wherein the relative positions areobtained relative to the magnetic field detecting element, and theinitial indicator values indicate the respective relative positions;obtaining converting means that converts the relationship to the linearfunction; and writing the converting means in the processor.
 5. Themethod according to claim 4, wherein the second magnet is a mastermagnet.
 6. The method according to claim 4, wherein the second magnet issame as the first magnet.
 7. The method according to claim 1, whereinthe indicator value S is an output voltage of the stroke sensor, whereinthe output voltage is based on an angle of the magnetic field, the anglebeing obtained from the magnetic field that is detected by the magneticfield detecting element.
 8. The method according to claim 1, wherein theindicator value S is an output voltage of the stroke sensor, wherein theoutput voltage is based on intensity of the magnetic field, theintensity being obtained from the magnetic field that is detected by themagnetic field detecting element.
 9. The method according to claim 1,wherein the second reference position is an initial position where thefirst magnet is attached to the structure.